1. Field of the Invention
The present invention relates to a pyroelectric thermal radiation sensor and more particularly to a pyroelectric thermal radiation sensor having two elements connected in parallel opposing fashion and mechanically mounted so as to null out vibration and to provide good low frequency response.
2. Description of the Prior Art
The use of pyroelectric sensors using polarized crystals and plastic films for detection of thermal radiation is well known. There have been many uses developed for such pyroelectric sensors. One important application is in the field of intruder alarms in which a pyroelectric sensor is arranged to detect the infrared radiation from a human being as an indication of an intruder. However, some prior art intruder alarms can be falsely triggered by other sources of infrared radiation such as hot air from a heating system, sunlight, or equipment which may give off heat radiation. To discriminate between an actual intruder and such environmental effects, it is known to require movement of the source of heat. For this purpose, it is common to utilize at least two collocated sensors in a differential connection so as to produce zero net signal output when both sensors are irradiated and to produce multiple pulse outputs when a person walks past the device. A typical example is shown in U.S. Pat. No. 3,839,640 to Rossin which teaches the use of two pyroelectric sensors formed from a single piece of polyvinylidene fluoride (PVF.sub.2) plastic film. This patent shows two such sensors connected in series opposition. There are several basic problems with the series opposition detector. One problem is that an output voltage produced by one of the sensors is required to cause a current to flow through the non-energized sensor which is of an extremely high impedance. Unfortunately, this greatly reduces the sensitivity of the device. Some series opposition type differential sensors utilize a single pyroelectric crystal or element which causes an additional loss of sensitivity. For example, the Rossin detector, which uses a single PVF.sub.2 film, has a floating rear electrode and two front electrodes such that a series opposition connection is obtained with respect to the two front electrodes. Since the PVF.sub.2 film is a good conductor of heat, heating of one sensor results in conduction of heat to the non-excited sensor. This generates a common mode signal which reduces the net output signal. It is also necessary to provide a backing for PVF.sub.2 film which can act as a heat sink which reduces the response to low frequency source (slow moving targets).
Pyroelectric crystals have been found to be much superior to pyroelectric PVF.sub.2 films since the plastic film is flimsy and easily damaged. The output for a given area PVF.sub.2 sensor is much less than for crystal types. Liu, in U.S. Pat. No. 3,816,750, discloses a series opposing connected differential sensor utilizing a polarized crystal which, although producing a higher output than the PVF.sub.2 type, suffers from the reduction in sensitivity common to this connection. As is well known, the resistivitiy of the inorganic pyroelectric crystals is very high. When the common electrode is left floating, electrostatic charges can build up on one or the other element which can cause random noise and biasing problems when directly coupled to field effect transistor (FET) type amplifiers.
It is also known in the prior art to connect two pyroelectric sensors in parallel opposition. Such connection will also reject signals produced by radiation common to both elements and has the advantage of a much higher sensitivity and output when only one sensor is energized. Such units also are free of the static build up problem. Typical of this type of device are the structures disclosed in U.S. Pat. No. 3,877,308 to Taylor and of McHenry in U.S. Pat. No. 3,453,432. Each disclosures a pyroelectric radiation detector having at least two parallel opposed connected sensors. McHenry teaches one sensor especially adapted to detect incoming radiation and the second sensor shielded from incoming radiation such that the pair of sensors will be compensated for environmental temperature changes.
A basic problem with all pyroelectric detectors, either crystal or plastic film, is that they also tend to be piezoelectric as well as pyroelectric. False signals can be generated due to vibration or other bending forces applied to such sensors. This characteristic can result in false alarms in intrusion detection applications.
The mounting of pyroelectric crystals introduces problems when good low frequency response is required. For example, the McHenry differential detector is mounted on a heat sink. When a slowly varying thermal radiation is incident on the McHenry sensor, an initial temperature gradient will occur across the crystal, but, due to the very high heat conductivity of the crystal, heat will flow into the heat sink, greatly reducing the gradient and therefore the output voltage. Taylor discloses only a sensor and does not consider the mounting problem. As previously mentioned, it is not desirable to utilize a single crystal having dual electrodes since the common mode created by heat flow from one element to the other reduces sensitivity. However, the use of two separate detectors in a differential sensor introduces the problem of false signals due to vibrations and the piezoelectric effect. Thus, there is a need for a high output differential thermal radiation sensor having common mode cancellation for both pyroelectric and piezoelectric signals and that will be sensitive to slowly varying thermal radiation.